Methods and ink compositions for placing indicia on glass are well known in the art. Typically, ink in the form of a paint or paste is applied to the surface of a glass article in a predetermined pattern, then dried to form indicia on the surface of the glass. Surface applied paints, however, are susceptible to damage due to abrasion, chemical attack, thermal cycling of the glass article, etc.
Indicia may permanently be affixed to glass utilizing thermal diffusion dyes containing metallic compounds such as, for example, silver, gold, or copper salts. Generally such thermal diffusion dyes are in the form of a viscose paint, or paste containing a metallic salt, which is applied to the surface of a glass article in a predetermined pattern. After the paint has dried, the glass article, including the thermal diffusion paint thereon, is heated to a temperature and for a time, sufficient to cause the metallic ions contained in the paint to diffuse into the glass article where the metal ions impart color to the glass. For example, copper diffusion dyes produce a red coloration in glass. Thus, the glass receives permanent indicia having a pattern substantially identical to the predetermined pattern of the originally applied thermal diffusion dye. Such indicia have superior chemical resistance and thermal stability, and are not susceptible to damage by abrasion.
The thermal diffusion dyes known in the prior art are generally applied as relatively thick, viscose layers on the surface of a glass article by brushing, spreading, or screen printing. These dye compositions must be highly viscose in order to retain their initially applied configuration through the drying and heating steps. Such highly viscose dyes would not be suitable for use in certain application devices such as, for example, a non-contact ink jet printer. However, the use of such a printer for applying a thermal diffusion ink to glass would be highly desirable due to its ease of operation, precise pattern repeatability, and flexibility, allowing instant modification of the printed pattern. For example, a non-contact ink jet printer could easily be adapted to apply consecutive numbers to the surfaces of consecutively produced glass articles, whereas a similar method for applying consecutive numbers in the form of surface applied indicia may only be accomplished by changing masks when using a screen printing operation.
Kume U.S. Pat. No. 4,056,643 discloses a method and dye composition for staining the entire surface of a glass article, to produce a granite-like mosaic decorative design having areas which appear irregular in size and shape of coloration, and which vary in shading and variety of colors. A staining composition, comprising at least one of a silver compound such as, for example, silver nitrate, and a copper compound, together with a vanadium compound capable of forming vanadium pentoxide by its subsequent heat treatment, is applied to the entire surface of a glass article. Thereafter, the glass article is heated to a temperature and for a period of time sufficient to obtain the desired degree of granite-like coloration and visual texture. It is theorized that the vanadium compound is converted to vanadium pentoxide which forms a melt together with the silver or copper compound on the surface of the glass. The melt forms an assemblage of droplets having irregular sizes and concentrations of the silver or copper compound. This non-uniform distribution results in the variation of coloration and shading in the ultimately produced stained glass article. Such a process would not be useful, however, for affixing precisely reproducible indicia to portions of the surface of a glass article.
Kiefer U.S. Pat. No. 3,907,586 discloses glass articles which are colored brown to reddish-brown over their entire surfaces. Borosilicate glass, free from arsenic and antimony, is melted along with 0.005 to 0.5 weight percent tin, to form glass articles which thereafter may be stained by the application and heat treatment of a silver diffusion dye. The tin introduced into the melt is necessary to reduce the silver ions which subsequently diffuse into the glass. The patent also discloses that it is known to add silver to a soda-lime-silica glass melt, to produce glass having a yellow color. The patent, however, neither teaches nor suggests a method for permanently affixing indicia to a portion of the surface of a glass article.
Ball U.S. Pat. No. 4,835,208 discloses the use of a non-contact ink jet printer for placing surface indicia on glass, and hot melt ink compositions for use therein. The disclosed ink compositions are said to solidify rapidly on the cooler surface of the glass substrate, to give a sharp image which is resistant to smudging. The patent does not, however, suggest the use of thermal diffusion inks containing silver nitrate, and teaches away from the step of heating the glass article after the hot melt ink indicia has been applied which would cause the hot melt ink to remelt and run.
It must be noted that the prior art referred to hereinabove has been collected and examined only in light of the present invention as a guide. It is not to be inferred that such diverse art would otherwise be assembled absent the motivation provided by the present invention. The prior art teaches that ink compositions for placing indicia on glass surfaces must be highly viscose in order to retain their as-applied configuration or pattern during subsequent fixing treatments which generally require heating of the glass article. Such a heating step would otherwise cause a predetermined pattern formed from a low viscosity material to run or smear. Thus, the prior art suggests that very low viscosity inks are not to be used for placing indicia on glass. Furthermore, the highly viscose inks taught in the prior art cannot be used in conjunction with a non-contact ink jet printer, due to the inherent limitations on viscosity which must be observed to insure the stable formation of ink droplets and to prevent plugging of the small diameter printer nozzels. Finally, although the prior art teaches that indicia may be placed on glass either by screen printing a highly viscose paint or by ink jet printing a lower viscosity hot melt ink onto the surface of the glass, such indicia could permanently be affixed to the glass, if either composition contained a thermal diffusion dye, only by heating the glass to a very high temperature. Such a heating step might successfully be performed on a glass article having screen printed indicia thereon. However, the prior art suggests that such a heating step would destroy lower viscosity hot melt ink indicia applied by an ink jet printer. Since hot melt indicia are rapidly solidified or frozen onto the cooler surface of the glass article, any subsequent heating step would cause the applied indicia to run. Thus, the prior art suggests that an ink jet printer may not be used to permanently affix indicia to glass where the process for doing so includes the step of heating the glass article.
It would be desirable to prepare a thermal diffusion ink composition which is useful for permanently affixing indicia to glass, having a low viscosity for use in a non-contact ink jet printer. The thermal diffusion dyes presently known in the art as useful for placing indicia on glass are highly viscose materials which are generally applied to the surface of a glass article by a screen printing process, and therefore would not be operable in an ink jet printer. Moreover, it would be desirable to permanently affix indicia to glass, utilizing a non-contact ink jet printer and silver nitrate-containing thermal diffusion ink which is thereafter heat treated without destroying the predetermined pattern to cause silver ions to diffuse into the surface region of the glass article. The prior art suggests the use of conventional hot melt ink compositions in conjunction with an ink jet printer, to place indicia, which would be susceptible to abrasion and corrosive attack, on the surface of glass. The prior art does not suggest, however, the use of a silver-containing thermal diffusion ink in such a device, which is applied to a glass surface and thereafter heat treated to penetrate and permanently affix indicia to a glass article.